1. Field of the Invention
The present invention relates to an inhomogeneously broadened transition (IBT) material, also called a spatial-spectral (S2) material, to achieve optical analog signal processing, and in particular to using multiple frequency chirp segments to read out an absorption spectrum stored in the material.
2. Description of the Related Art
The absorption features of ions or molecules doped into inorganic or organic materials are spectrally broadened by two main classes of mechanisms. Homogeneous broadening is the fundamental broadening experienced by all ions or molecules independently, and arises from the quantum-mechanical relationship between the frequency line shape and the dephasing time of the excited electron in the ion or molecule. Inhomogeneous broadening refers to the quasi-continuum of overlapping individual spectra of all of the ions or molecules in the material, which have microscopically different environments and therefore slightly different transition frequencies. When the inhomogeneous broadening of a material is significantly larger than the homogeneous broadening of a material, the material displays useful optical absorption properties and is called an IBT material. Doping of certain rare earth ions in inorganic materials in a certain way produces IBT materials that are useful in broadband signal processing applications. In various materials known in the art, the absorption demonstrates optical frequency selectivity over bandwidths typically far greater than 1 gigaHertz (GHz, 1 GHz=109 Hz, Hertz=cycles per second) and with frequency resolution typically far less than 1 megaHertz (MHz, 1 MHz=106 Hz).
The frequency selectivity can be modified locally by interaction with optical signals that excite electrons in the ions, which serve as absorbers, from a ground state to an excited state, thereby removing those electrons from the population of ground state absorbers at that location in the material. This creates a reduction in the absorption at the resonant frequency of these ions. Therefore, some such materials have been used to form highly frequency selective spatial-spectral gratings, and these materials are sometimes called spatial-spectral materials (S2 materials). After some time, the electrons may return to the ground state and the grating decays with a characteristic time called the population decay time. When electrons are removed from the ground state in a particular homogeneously broadened absorption peak, a “hole” is said to be “burned” in the absorption of the material at the frequency of the hole, and light at the frequency of the hole is transmitted with substantially less absorption. A spectral hole is an example of the most simple burned spectral feature, and combinations of spectral holes at different frequencies with varying depths are here denoted as spectral features or spectral gratings. The lifetime of the spectral features is determined by the time it takes for the absorbers in the system to return to their equilibrium state. Spectral features may be made permanent in some systems.
Some IBT materials have been used as versatile optical coherent transient (OCT) processing devices. An OCT device relies on a broadband spatial-spectral grating in the optical range that extends over several homogeneous lines, and part or all of the available inhomogeneous broadening absorption profile. All the features of an optical spatial-spectral grating are typically formed substantively simultaneously by recording the spatial spectral interference of two or more optical pulses separated in time only (purely spectral grating) or separated in both space and time (a spatial-spectral grating). A spatial-spectral grating has the ability to generate a broadband optical output signal that depends on an optical input probe wave form impinging on that grating and the programming pulses that formed the grating.
In some OCT devices, the approach to accessing the information in the spatial-spectral grating is to probe that grating with a high bandwidth, Fourier-transform-limited optical signal, such as a coherent brief optical pulse, or a series of such coherent brief optical pulses. Under certain conditions the probing of the grating can produce optical output signals that are generally referred to as stimulated photon echoes or optical coherent transients. A single brief coherent light pulse with a bandwidth equal to that of the spectral grating stimulates a time-delayed output signal whose temporal profile represents the Fourier transform of the spectrum recorded in the grating structure.
While useful in many applications, the approach of readout with a high bandwidth, Fourier-transform-limited coherent brief optical pulse or series of optical pulses at the full bandwidth of processing can suffer, at present, from the limited performance in dynamic range of photo-detectors and analog to digital converters (ADCs) that are needed to make a measurement of any instantaneous high bandwidth optical signal. Existing high bandwidth detectors and ADCs (also called “digitizers” herein) have limited performance and higher cost as compared to lower bandwidth detectors and digitizers. For example, currently available photodetectors with a bandwidth response of greater than 10 GHz (e.g., 1554 12-GHz photodetector, available from New Focus of San Jose, Calif.) has less than 30 dB of dynamic range. Much higher dynamic ranges of photodetectors are preferred for accurate measurement applications, which can be found in photodetectors with much lower (e.g. 1000 times lower) bandwidth responses. For example, photo-detectors with bandwidths on the order of 10 MHz can provide on the order of 90 dB of dynamic range (e.g., S2386 Silicon Photodetector, available from Hamamatsu Photonics, K.K., headquartered in Hamamatsu City, Japan, with offices worldwide).
Likewise, digitizers with sample rates over 1 giga-sample per second (Gs/s) are limited in performance and expensive. It is industry standard for a digitizer to produce 2.5 samples per resolved oscillation, so for example, a detector with an 8 GHz bandwidth would be followed by a digitizer with ˜20 Gs/s capability. Digitizer performance is specified in terms of N bits, where the analog signal can be quantized to one of ˜2N levels. A survey performed in 1999 of digitizer performance at that time found that the signal-to-noise ratio (SNR) bits, denoted as ‘SNR-bits’ of digitizers, expressed as effective number of bits according to SNR-bits=(SNR(dB)−1.72)/6.02, fell off with a slope of (−1) bit per octave of the sample rate (see R. H. Walden, “Analog-to-Digital Converter Survey and Analysis”, IEEE Journal on selected Areas in Communications, VOL. 17, NO. 4,1999). For example, a 100 M/s digitizer may have 11 SNR-bits, while a 12800 Ms/s (12.8 Gs/s) digitizer would have 4 SNR-bits. Even in 2004, after the invention described herein, the highest reported performance in SNR-bits at high sample rates was 3 to 4 SNR-bits at 20 to 40 Gs/s (see W. Cheng et. al, “40 GSPS ADC-DAC Components for the ADAM Receiver-Exciter ASIC” and F Stroili, et. al, “Multifunction Receiver-on-Chip Technology for Electronic Warfare Applications”, both articles found in the Proceedings of the GomacTech conference, Monterey, Calif., 2004). Present day oscilloscope technology offers effective digitizer performance with a bandwidth limit of 6 GHz (achieved with several ADC boards that together give an effective advertised sample rate of 20 Gs/s) and an effective resolution that is currently limited to about 3.5 bits at 6 GHz (e.g., TDS6604 real time oscilloscope, available from Tektronix of Beaverton, Oreg., USA). Much higher digitizer performance is preferred. In 2004, digitizers with operation sample rates on the order of 100 Mega-samples per second (Ms/s) can have up to 16 bits of resolution for a modest cost (e.g., AD10678, 16 bit digitizer at 80 Ms/s, available from Analog Devices of Norwood, Mass. for about $500). By means of cost comparison, a 10 GHz photodetector is about 100 times more expensive than a 10 MHz photodetector, and a 20 Gs/s oscilloscope (comprised of several digitizers and supporting electronics) is about 100 times more expensive than a ˜100 Ms/s digitizer (with supporting electronics).
Optical linear frequency modulation (LFM) signals, i.e., frequency “chirped” signals (also called “chirps”), have been used as waveforms in pulse sequences to write spatial-spectral gratings for applications of storage, signal processing, true time delay generation, and arbitrary waveform generation, and also for readout of spectral gratings. In the case of readout chirps used to generate coherent transient output signals, the chirps are typically limited in duration to less than the decoherence time of the transition These chirped probes generate a temporal output signal that represents a collective readout of all the absorbers, as with the brief pulse excitation, but under the condition of swept excitation.
In a recent approach, a temporally extended chirp is used as a probe waveform to generate a readout signal that represents a temporal map of the structure of the spectral population grating, rather than its Fourier transform as with a brief pulse. This readout signal can be measured with inexpensive, high dynamic-range, MHz bandwidth photodetectors and digitizers. Such extended chirps generally have a duration greater than the decoherence time and less than the population decay time of the inhomogeneously broadened absorption spectrum in the IBT material. This approach is described in patent application PCT/US03/14612 entitled “Techniques For Processing High Time-Bandwidth Signals Using A Material With Inhomogeneously Broadened Absorption Spectrum”, filed May 12, 2003 by inventors Kristian Doyle Merkel, Zachary Cole, Krishna Mohan Rupavatharam, William Randall Babbitt, Kelvin H. Wagner, and Tiejun Chang (hereinafter “Merkel”) and published by the World Intellectual Property Organization as WO 03/098384 A2 on 27 Nov. 2003, the entire contents of which are hereby incorporated by reference as if fully set forth herein. This approach is subsequently described in U.S. patent application Ser. No. 10/515089, filed Nov. 12, 2004.
As described in Merkel, an extended chirp sweeping over the entire IBT frequency band of interest, e.g., in excess of 1 GHz, can produce a readout signal with a low-bandwidth (a few to tens of MHz) intensity modulation that can be detected and digitized with the low-bandwidth high-dynamic-range devices that are currently available. This low-bandwidth readout signal represents a temporal map of the frequency spectral features in the spatial-spectral grating. For example, in some cases the readout signal can include temporal spikes that each represents a spectral hole burned in the IBT material. In other cases the readout signal includes a superposition of low-bandwidth beat frequencies, each beat related to a periodic component in the frequency spectrum of the grating. Multiple combinations and mixtures of these two example cases can be employed in some embodiments.
However, current known techniques for producing optical chirps that have the desired levels of linearity and stability in frequency are limited to using radio frequency pulses with bandwidths of about 1 GHz to drive optical modulators that modulate a frequency stable laser. Therefore, techniques are needed to generate a stable, linear frequency chirped pulse with greater than about 1 GHz bandwidth. Additionally, or in the alternative, techniques are needed to generate a low-bandwidth readout signal that represents a temporal map of the entire frequency band of interest in the IBT material, e.g., much greater than 1 GHz, using the currently available frequency chirped pulses of about 1 GHz.
Based on the foregoing, there is a clear need for techniques to generate a readout signal that represents an accurate, precise and substantively complete temporal map of the structure of the entire frequency band of interest in the IBT material, and which does not suffer the disadvantages of prior art approaches.
In particular, there is a clear need for generating a readout signal that represents an accurate, precise and substantively complete temporal map of about one GHz of bandwidth or more of the spectral content in IBT materials, which can be measured with inexpensive, high-dynamic-range, low-bandwidth detectors and digitizers.
Furthermore, there is a clear need for generating a readout signal that represents an accurate, precise and substantively complete temporal map of the entire spectral content of interest in IBT materials using several linear, stable chirps that each has a bandwidth smaller than the bandwidth of the spectral content of interest.
The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not to be considered prior art to the claims in this application merely due to the presence of these approaches in this background section.